Federal regulations of HC, CO and NOx have been in place for some time. Only recently has regulation of so called greenhouse gases, in particular CO2, been proposed. CO2 is directly related to fuel economy and engine efficiency and does not require any additional emission devices. Regulation of N2O, another alleged greenhouse gas, which has recently been proposed, presents new issues.
The inventors herein have recognized that N2O is most likely to be created in the low temperature exhaust gas aftertreatment systems of diesel engines and hybrid-diesel engines in particular. In such aftertreatment systems, an oxidation catalyst or DOC is placed in the engine exhaust to oxidize HC and CO over a catalytic surface typically containing precious metals such as platinum and palladium. A selective reduction base metal catalyst, or SCR, is placed downstream of the DOC and connected to an ammonia injection system which uses an ammonia-based reductant such as aqueous urea that is injected under certain engine operating conditions to chemically reduce NOx with ammonia.
More specifically, the inventors have recognized that HC from the engine may react with NOx in the DOC to produce N2O when the DOC is operating in a temperature window sufficiently high for the reaction to occur but sufficiently low such that not all the HC is oxidized. If diesel fuel is used to heat the DOC, then even more N2O would be produced with the additional HC. Another N2O reaction may occur in the SCR by reacting ammonia with NOx.
In one particular aspect of the invention, the inventors have addressed the issue of N2O generation in a method where exhaust gases from the engine are routed through an oxidation catalyst which includes a washcoat with palladium or platinum or both. N2O formation from the oxidation catalyst is inferred from one or more of, temperature of the catalyst, ratio of HC to NOx or ratio of NO2 to NOx in the engine exhaust gases; and reducing the N2O formation when the catalyst is operating within a temperature window associated with N2O formation. In a further aspect, the N2O formation is reduced by heating the catalyst from an external source. In this way, the technical result is achieved, and the catalyst is preferably heated by an electric heater and the heating ends when the catalyst temperature rises above a temperature range associated with N2O generation.
In still a further aspect, the reducing of N2O formation comprises increasing the effective compression ratio of the engine to reduce HC formation by the engine when the inferred N2O formation exceeds a predetermined value. The engine compression ratio increase comprises at least one of the following: changing intake valve timing of the engine, increasing pressure of air forced into the engine, or decreasing volume of combustion chambers of the engine.
In another aspect of the invention, the method includes routing exhaust gases from the engine through an oxidation catalyst which includes a washcoat with palladium or platinum or both; routing exhaust gases from the oxidation catalyst into a selective reduction catalyst; adding ammonia to the selective reduction catalyst under predetermined conditions to reduce NOx; inferring N2O formation from the oxidation catalyst from, temperature of the oxidation catalyst, HC and NOx and NO2 in the engine exhaust gases; inferring N2O out of the selective reduction catalyst from temperature of the selective reduction catalyst, the inferred N2O formation from the oxidation catalyst, and the ammonia; and heating the oxidation catalyst from an external source to reduce the N2O out of the selective reduction catalyst when the oxidation catalyst temperature is below a predetermined range and the N2O out of the selective reduction catalyst exceeds a preselected amount. Preferably, the inferred N2O from the oxidation catalyst is inferred from temperature of the oxidation catalyst, and the ratio of HC to NOx and the ratio of NO2 to NOx in the exhaust gases.
In still another aspect the invention is applied to a hybrid-diesel engine. In this aspect the method practiced comprises: routing exhaust gases from the engine through an oxidation catalyst which includes a washcoat with palladium or platinum or both; routing exhaust gases from the oxidation catalyst into a selective reduction catalyst; adding ammonia to the selective reduction catalyst under predetermined conditions to reduce NOx; inferring N2O formation from the oxidation catalyst from, temperature of the catalyst, ratio of HC to NOx and NO2 in the engine exhaust gases; inferring N2O out of the selective reduction catalyst from temperature of the selective reduction catalyst, the inferred N2O formation from the oxidation catalyst, NOx from the diesel engine, and the ammonia; heating the oxidation catalyst with an electric heater to reduce the N2O formation from the oxidation catalyst when the oxidation catalyst temperature is below a predetermined range and the inferred N2O out of the selective reduction catalyst exceeds a preselected amount; and discontinuing the electric heating when the oxidation catalyst temperature exceeds the predetermined range.
In a more specific aspect, power output of the diesel engine is reduced when the inferred N2O out of the selective reduction catalyst is above a predetermined value. Further, the electric motor is controlled to provide power related to the reduction in power from the diesel engine.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings. It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.